Various types of hearing prostheses may provide persons with different types of hearing loss with the ability to perceive sound. Hearing loss may be conductive, sensorineural, or some combination of both conductive and sensorineural. Conductive hearing loss typically results from a dysfunction in any of the mechanisms that ordinarily conduct sound waves through the outer ear, the eardrum, or the bones of the middle ear. Sensorineural hearing loss typically results from a dysfunction in the inner ear, including the cochlea where sound vibrations are converted into neural signals, or any other part of the ear, auditory nerve, or brain that may process the neural signals.
Persons with some forms of conductive hearing loss may benefit from hearing prostheses, such as acoustic hearing aids or vibration-based hearing devices. An acoustic hearing aid typically includes a small microphone to detect sound, an amplifier to amplify certain portions of the detected sound, and a small speaker to transmit the amplified sounds into the person's ear.
Vibration-based hearing devices typically include a small microphone to detect sound and a vibration mechanism to apply vibrations corresponding to the detected sound directly or indirectly to a person's bone or teeth, thereby causing vibrations in the person's inner ear and bypassing the person's auditory canal and middle ear. Vibration-based hearing devices include, for example, bone conduction devices, direct acoustic cochlear stimulation devices, or other vibration-based devices. A bone conduction device typically utilizes a surgically implanted mechanism or a passive connection through the skin or teeth to transmit vibrations corresponding to sound via the skull. A direct acoustic cochlear stimulation device also typically utilizes a surgically implanted mechanism to transmit vibrations corresponding to sound, but bypasses the skull and more directly stimulates the inner ear. Other non-surgical vibration-based hearing devices may use similar vibration mechanisms to transmit sound via direct or indirect vibration of teeth or other cranial or facial bones or structures.
Persons with certain forms of sensorineural hearing loss may benefit from hearing prostheses, such as cochlear implants and/or auditory brainstem implants. For example, cochlear implants can provide a person having sensorineural hearing loss with the ability to perceive sound by stimulating the person's auditory nerve via an array of electrodes implanted in the person's cochlea. A microphone of the cochlear implant detects sound waves, which are converted into a series of electrical stimulation signals that are delivered to the implant recipient's cochlea via the array of electrodes. Auditory brainstem implants can use technology similar to cochlear implants, but instead of applying electrical stimulation to a person's cochlea, auditory brainstem implants apply electrical stimulation directly to a person's brain stem, bypassing the cochlea altogether. Electrically stimulating auditory nerves in a cochlea with a cochlear implant or electrically stimulating a brainstem may enable persons with sensorineural hearing loss to perceive sound.
Further, some persons may benefit from hearing prostheses that combine one or more characteristics of the acoustic hearing aids, vibration-based hearing devices, cochlear implants, and auditory brainstem implants to enable the person to perceive sound. Such hearing prostheses can be referred to generally as bimodal hearing prostheses. Generally, the term bimodal means more than one stimulation mode, and not necessarily only two stimulation modes.
The effectiveness of a hearing prosthesis depends on the design of the prosthesis itself and on how well the prosthesis is configured for or fitted to a prosthesis recipient. The fitting of the prosthesis, sometimes also referred to as programming or mapping creates a set of configuration settings and other data that define the specific characteristics of the signals (acoustic, mechanical, or electrical) delivered to the relevant portions of the person's outer ear, middle ear, inner ear, auditory nerve, or skull. This configuration information may also include a prescription rule that defines a relationship between audio input parameters and output parameters for audio frequency channels of the hearing prosthesis.
Referring more particularly to acoustic hearing aids, an example prescription rule can include applying frequency shifting to process incoming sound before applying amplified sounds into the person's ear. Frequency shifting in the context of acoustic hearing aids is performed primarily to move sound information from impaired higher frequency regions of the cochlea to better functioning lower frequency regions of the cochlea.